Signal processing to enable utilization of a rig reference sensor with a drill bit seismic source

ABSTRACT

An apparatus (10) for providing while drilling information on a subterranean geologic formation (28) includes a drilling rig (12) and a rotary drill bit (18) attached to the drilling rig (12) for providing seismic waves as it drills in the earth (16). Geophones (20) are spaced from the rotary drill bit (18) in the earth (16) and receive indirect seismic wave paths (26) and seismic wave paths (30) reflected from the subterranean geologic formation (28) the seismic waves provided by the drill bit (18). A reference sensor (24) is located on the drilling rig (12). The seismic signals sensed by the reference sensor (24) and by the geophones (20) are cross-correlated to separate the drill bit generated signals from interference signals by combining the reference signals and the signals received by the geophones (20). The cross-correlated reference signals and the signals received by the geophones (20) are separated into a first group of the drill bit generated seismic signals travelling to the geophones (20) in direct paths and a second group of the drill bit generated seismic signals travelling to the geophones (20) in paths reflected from the subterranean geologic formation (28). The cross-correlation provides a domain where drill bit generated energy can be distinguished from interference. The two groups of the drill bit generated seismic signals can then be used to image the subsurface seismically so as to improve the chances of discovering hydrocarbons. The techniques is applicable to land operations and to drilling from an offshore platform over water.

This is a continuation of application Ser. No. 260,784 filed Oct. 21,1988, now U.S. Pat. No. 4,926,391.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention concerns an apparatus and method by which the randomacoustic vibrations emanated by a drill bit while drilling can be usedto create seismic images of the subsurface in a region in the vicinityof the borehole. More particularly, it relates to such an apparatus andmethod in which sensors are positioned to provide more completeinformation on subterranean geologic formations. Most especially, itrelates to such an apparatus and method in which improved referencesignals and improved signal processing techniques are available.

2. Description of the Prior Art

Conventional reflection seismology utilizes surface sources andreceivers to detect reflections from subsurface acoustic impedancecontrasts. The obtained image often suffers in spatial accuracy,resolution and coherence due to the long travel paths between source,reflector and receiver. It is well known to seismologists that therecorded signal amplitude at the receiver, A(r), is related to the inputsignal amplitude, A_(o), through the travel-path by: ##EQU1## whereα=The absorption coefficient of the earth along the travel-path

r=The travel path length

Longer travel paths have lower recorded amplitudes.

A technique commonly known as Vertical Seismic Profiling (VSP) recordsseismic data to image the subsurface in the vicinity of the borehole.With VSP a source(s) is activated at the surface and a sensor(s) issuspended at a discrete borehole depth with a wireline. Data is recordedand the process is repeated for several borehole depths. Acquiring VSPdata is highly impractical if multiple surface source locations aredesired. Each new source is costly to apply and adds to the acquisitiontime as well as the rig inactivity period. A Vertical Seismic Profile istherefore prohibitively expensive to implement when may source positionsare used.

With a downhole source, the VSP geometry is inverted. Source locationsbecome receiver locations and vice versa. A downhole source overcomesthe logistical limitations inherent in using multiple surface sourcesfor Vertical Seismic Profiling. Data can be acquired simultaneously atmany sensor locations proximal to the Earth's surface with little moreexpense than a single location.

One of the earliest patents concerning downhole sources is Weatherby,U.S. Pat. No. 2,062,151, issued Nov. 24, 1936, which uses the drill bitas an impulse generator of seismic waves. Drilling is done with a cabletool, which is dropped on the hole bottom. This creates seismicimpulses. Bit location and wave velocity can be obtained using theseimpulses. Drill bit generated direct wave arrival time differencesbetween two non coincidental geophone locations are used to determinerock acoustic velocity The Widrow U.S. Pat. Nos. 4,363,112 and4,365,322, issued Dec. 7 and Dec. 21, 1982, use the continuous, natural,random vibrations of a rotary drill bit to launch seismic waves into theearth. Spectral amplitudes and interference patterns are used to imagesubsurface reflectors.

There are a number of previous patents concerned with the use of seismicdownhole sources other than the drill bit. All of these patents make useof an artificial transducer situated at a given depth in the borehole.Broding et al., U.S. Pat. No. 3,909,776, issued September 1975 and Farrand Ward, U.S. Pat. No. 3,881,168, issued April 1975, use a fluid drivenoscillator (as described by Galle in U.S. Pat. Nos. 3,520,362,3,730,269, 3,860,902 to emit seismic waves into the earth from aposition within a wellbore. Farr and Ward require that the seismicsource be monofrequency. Phase delays between a geophone located on thesurface near the well and another located near the top of the well areused to produce a log of travel-time and compressional wave velocity asa function of depth. Broding et al. use a fluid driven oscillator whichchanges emitted frequency as a function of time, much like a Vibroseissource, as described in U.S. Pat. No. 2,688,124. The repeatable patternof the oscillator can then be used as a signal to cross-correlate withreceived geophone signals. Arrival times of events as determined bypeaks in the cross-correlations can then be used to seismically imagethe subsurface.

The patent literature describes other types of downhole seismic sourceswhich emit pulses into the subsurface. For example, Klaveness, U.S. Pat.No. 4,207,619, issued June 10, 1980, uses a pulse generator located inthe drillstring just above the drill bit. The source is activated andthe arrival time measured at the earth's surface and at the top of thedrillstring on the swivel. The swivel location is used as a "zero point"sensor from which pulse arrival time differences can be measured.Arrival times are calculated from visual inspection of time-domainsignals received by the geophones.

While the art pertaining to seismology is clearly a well-developed one,a need still remains for further development of it in order to meet thedemands of gas, oil and other resource exploration.

SUMMARY OF THE INVENTION

Accordingly, it is an object of this invention to provide an apparatusand method for providing information on a subterranean geologicformation in which sensors for drill bit generated seismic waves arepositioned to make improved use of a reference signal to provide thedesired information.

It is another object of the invention to provide such an apparatus andmethod in which the desired information is obtained by improved signalprocessing made possible by the reference signal.

It is a further object of the invention to provide such an apparatus andmethod in which all of the signals used to obtain the desiredinformation originate from the drill bit.

It is still another object of the invention to provide such an apparatusand method which is non-invasive and generates the signals used toobtain the desired information while drilling.

It is a still further object of the invention to provide such anapparatus and method which will give real time subsurface information.

The attainment of these and related objects may be achieved through useof the novel apparatus and method herein disclosed. An apparatus forproviding while drilling information on a subterranean geologicformation in accordance with this invention includes a drilling rig anda rotary drill bit attached to the drilling rig for providing periodicseismic waves as it drills in the earth. There is at least one seismicwave sensor spaced from the rotary drill bit in the earth for receivingsignals traveling via direct seismic wave paths and signals travelingvia seismic wave paths reflected by the subterranean geologic formationfrom the seismic waves provided by the drill bit. At least one referencesensor is located on or proximate to the drilling rig. A means isconnected to receive the reference signal from the reference sensor andthe drill bit generated signals from the at least one seismic wavesensor to distinguish the drill bit generated signals from interferencesignals by cross-correlating the reference and seismic wave sensorsignals. In a preferred form, the apparatus has a means connected toreceive the reference signals either prior to or subsequent to theircross correlation for reference deconvolution or whitening. A means isconnected to receive the cross-correlated reference and seismic wavesensor signals for eliminating rig generated energy from the referencesignals. A means is connected to receive the cross correlated referenceand seismic wave sensor signals from the rig generated energyeliminating means for separating the seismic wave sensor signals into afirst group of the seismic wave sensor signals representing the drillbit generated seismic waves received by the at least one seismic wavesensor in the direct seismic wave paths, and a second group of theseismic wave sensor signals representing the drill bit generated seismicwaves received by the at least one seismic wave sensor in the seismicwave paths reflected by the subterranean geologic formation.

In the method for providing information on a subterranean geologicformation of this invention, seismic signals are provided by a rotarydrilling bit while drilling in the earth with the rotary drilling bit.The drill rig could be on land or on an offshore platform. The seismicsignals provided by the drilling bit are received via direct seismicsignal paths and seismic signal paths reflected from the subterraneangeologic formation in at least one location in the earth spaced from therotary drilling bit. The drill bit generated seismic signals arereceived as reference signals proximate to a drilling rig connected tothe rotary drilling bit. The seismic signals sensed proximate to thedrilling rig and spaced from the drilling rig are cross-correlated toallow the drill bit generated signals to be distinguished frominterference signals by combining the reference signals and the signalsreceived spaced from the drilling rig. The cross-correlated referencesignals and the signals received spaced from the drilling rig areseparated into a first group containing drill bit generated seismicsignals travelling to the at least one location spaced from the drillingrig in direct paths and a second group containing drill bit generatedseismic signals travelling to the at least one location spaced from thedrilling rig in paths reflected from the subterranean geologicformation.

Our apparatus and method is significantly different from the above priorart techniques. Like Weatherby and Widrow, we use the drill bit as agenerator of seismic waves. However, we employ very differentacquisition and processing techniques to exploit the drill bit generatedseismic energy. Techniques such as acquiring the seismic datasimultaneously at a reference location on the drill rig as well as withgeophones located in the earth, cross-correlation of the rig referencesensor signal with received geophone signals and associated processingsuch as deconvolution of the rig reference signal and attenuation of riggenerated interference improve over the Weatherby and Widrow techniques.

All of the prior art using downhole sources do not use the drill bitsignal to seismically image the subsurface They all require that anartificial transducer be located in the borehole. Some such as Farr andWard, and Klaveness, use signals from a sensor located at the top of thedrill pipe to aid in processing and interpreting the data. Broding etal. do not use a sensor located at the top of the drill pipe but do makeuse of cross-correlation techniques to determine arrival times ofevents. None of this prior art uses the cross-correlation functionbetween a rig sensor signal and geophone signals to measure arrivaltimes of events originating from the drill bit. None of the prior artteaches means .of separating reflected drill bit energy from directlytraveling drill bit energy and rig generated interference.

Drill bit generated seismic energy travels into the earth and togeophone locations on the earth's surface via many different travelpaths. Some of the major travel paths are illustrated in FIG. 1. Inaddition to direct and reflected paths, drill bit generated energy alsotravels up the drillstring to the drill rig. Some of this energycontinues from the drill rig into the earth and travels to thegeophones, introducing coherent interference. By cross-correlating thesignal from a rig mounted reference sensor with signals from geophoneslocated some distance from the borehole in the earth, relative arrivaltimes of events between signals from the rig sensor and the geophonescan be measured.

The rig reference signal causes complications in the cross-correlateddata not encountered when using conventional seismic sources. The rigreference signal has a highly colored spectrum and contains multiplesources of noise from the rig's drilling machinery. The rig referencesignal is further complicated by multipath and losses in thedrillstring. These problems are solved by the methods taught herein.Deconvolution can be used to whiten the spectrum of the rig referencesignal and reduce multipath effects and losses in the drillstring. Inaddition, coherent interference radiated by the drill rig can beattenuated through spatial filtering.

Using drilling data recorded from one or a number of field sensors, andhaving made these recordings at many drilling depths, methods are shownto create two and three dimensional seismic images. These methodsinclude wavefield separation and Common Reflection Point transformationand analysis.

The drill bit downhole source holds several advantages over proposed andexisting downhole sources. All downhole seismic sources are specialtransducers used to generate seismic waves. Such sources require thatthe rig remain idle for the period of time when they are activated.Their use poses risk to the borehole and causes loss of rig time. Theyare expensive and unreliable devices. In contrast, the drill bit sourceis passive, utilizing natural acoustic emanations of the drill bit whiledrilling. Use of the drill bit is non-invasive, having no effect on thedrilling process and introducing no additional risk to the borehole.Moreover, imaging with the drill bit source is a measure-while-drill(MWD) technique. The information provided by a survey based on thistechnique in real time may have a significant impact on various drillingrelated decisions. For example, it is often desirable to determine thelocation of specific geologic horizons relative to the drill bit depthin order to optimize drilling parameters. If an overpressured zone wereexpected, accurate knowledge of depths at which formationover-pressuring occurred would facilitate amendments to a drillingfluids program and reduce blow out risk. Conventional VSP methods ofoverpressure identification require that drilling be stopped to conductthe survey.

The subsurface seismic images constructed while drilling ma also aid theexplorationist in determining whether and/or when the target horizonswill be penetrated if the drilling plan is followed and whether the planneeds updating.

This invention teaches how to record and isolate the drill bit generateddirect and reflected seismic energy traveling in the earth from theseismic energy recorded at locations on and in the vicinity of the drillrig. The drill bit generated direct and reflected energy can then beused to obtain subsurface seismic images in the vicinity of theborehole.

The attainment of the foregoing and related objects, advantages andfeatures of the invention should be more readily apparent to thoseskilled in the art, after review of the following more detaileddescription of the invention, taken together with the drawings, inwhich:

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic representation of an apparatus in accordance withthe invention.

FIG. 2 is a flow chart of a process in accordance with the invention.

FIGS. 3A and 3B are waveform diagrams of signals obtained using theapparatus of FIG. 1.

FIG. 4 is another waveform diagram of further signals obtained using theapparatus of FIG. 1.

FIGS. 5A and 5B are additional waveform diagrams obtained after furthersignal processing of the signals shown in FIG. 3A.

FIG. 6 is a schematic representation useful for understanding furtheroperation of the apparatus of FIG. 1.

FIG. 7 is a set of signal waveforms illustrating the operation of theapparatus of FIG. 1 explained in connection with FIG. 6.

DETAILED DESCRIPTION OF THE INVENTION

Turning now to the drawings, more particularly to FIG. 1, there is showna seismic signal generation and analysis apparatus 10 in accordance withthe invention. The apparatus 10 includes a drilling rig 12, adrillstring 14 extending into earth 16 from the drilling rig 12, and adrill bit 18 at the end of the drillstring 14. A group of geophoneseismic sensing elements, generally indicated at 20, is provided in theearth 16, near its surface 22. A reference geophone seismic sensingelement 24 is mounted on the drilling rig 12, such as above the kelly.

In operation of the apparatus 10, the drill bit 18 generates seismicwaves while drilling a borehole, as is explained more fully in theabove-referenced Widrow patents. These seismic waves are propagatedthrough the earth 16 to the seismic sensing elements 20 both directly,as indicated at 26, and by reflection from one or more subterraneangeologic formations 28, as indicated at 30. The seismic waves arefurther propagated along the drillstring 14 to the reference sensingelement 24 on the drilling rig 12. Interference signals are generated bythe drilling rig 12, and are also propagated to the seismic sensingelements 20, as indicated at 32, and also to the reference sensingelement 24.

The resulting seismic energy is recorded by the reference vibrationsensor 24 located on the drill rig and simultaneously at the fieldsensors (geophones) 20 located in the earth 16 at selected distancesfrom the borehole. The seismic energy consists not only of energyemanated by the drill bit 18, but also other components of the drillingapparatus such as diesel engines, compressors, etc. The sound recordedby field sensors 20 contains not only the signals associated withdrilling, but also components which may be unrelated to drilling Thelatter include any cultural noise such as vehicles, people, animals,weather (wind and rain), etc. The vibrational energy recorded by thesensor 24 on the swivel or on the drill rig is used as a referencesignal with which to cross-correlate the geophone outputs.Cross-correlation, as described by Doty in U.S. Pat. No. 2,688,124,provides means by which arrival times of energy which is coherentbetween the reference signal and the geophone outputs can be measured.Energy which is incoherent is not sensed at both reference and fieldlocations and is attenuated in the cross-correlation process.

Our method utilizes a downhole seismic source, acoustic energy generatedby the drill bit 18 while drilling, and traveling through the earth 16via direct and reflected paths 26 and 30 to seismically image thesubsurface near the borehole. Seismic data is recorded while a well isbeing drilled using sensors 20 in the earth in the vicinity of theborehole and on the drill rig 12.

The drill bit signal is an uncontrolled random process. The reference onthe drill bit 18 source is obtained remotely by exploiting the vibrationfrom the drill bit 18 arriving at the surface via the drillstring 14.Specialized signal processing as outlined below is used to effectivelyrepresent the cross-correlated data as if the source spectrum werecontrolled and the sensor recording the reference signal were locatedproximal to the drill bit.

Our method holds several advantages over surface reflection seismology.With our method, the travelpath to a reflector 28 is reduced by as muchas a factor of 2 from surface reflection seismic paths. It has also beenobserved that the near surface acts as a strong attenuator of seismicenergy, particularly at frequencies above 50 Hz. Using the drill bit 18signal, the effects of the near surface are halved. Drill bit generatedseismic energy arrives via direct as well as reflected travel-paths 26and 30. Direct arrivals are stronger than reflections and hence can beexpected to exhibit a higher signal to noise ratio.

Our method, using the drill bit 18 as a downhole source, overcomes thelogistical limitations inherent in using multiple surface sources forVertical Seismic Profiling. Our data can be acquired simultaneously atmany field sensor 20 locations with little more expense than at a singlesurface location.

FIG. 2 is a flow chart showing how the reference signals 25 generated atthe drill rig 12 and the field geophone signals 27 are processed topractice the method of this invention. The reference signals aredeconvolved at 29 to whiten them. The deconvolved reference signal iscross-correlated with the field geophone signal 27 at 31. As part of thecross-correlation, the signal 25 is time shifted, as indicated at 33.These steps are repeated for a series of drilling depths, as indicatedat 34. The time shifting could occur at any point in the signalprocessing. The output of the cross-correlation 31 at 35 is a singlesignal with elements of both input signals 25 and 27. This process isrepeated for selected drilling depths. The signals are then sortedaccording to ascending drilling depths at 36. Rig generated energy isremoved from the signal 35 at 37. The signal containing mixed direct andreflected drill bit generated seismic components is then separated at 39to give the drill bit generated direct arrival component at 41 and thedrill bit generated reflected component at 43. Analysis of these twosignal components characterizes the subterranean geologic formation fromwhich the component 43 was reflected. The direct arrival component 41 isused at 44 to determine propagation velocity. The direct arrivalcomponent 41 is used at 46 to calculate or derive a deconvolutionoperator. The deconvolution operator 46 is used at 48 to deconvolve thedrill bit generated reflected component 43. The deconvolved drill bitgenerated reflected component is then used at 50 for common reflectionpoint imaging.

REFERENCE DECONVOLUTION

Seismic vibrations from the drill bit 18 are mechanically transmittedthrough the drill collars and through the drill pipe to the top of thekelly and into the swivel. Hence, the reference signal recorded on therig 12 at the top of the drillstring corresponds to the bit generatedsignal modified by the transfer function between the bit 18 and thereference 24. Both coherent noise generated by the drilling machineryand random noise are present at the reference 24. In Z-transformnotation:

    Ref(Z)=Bit(Z)T(Z)+N(Z)

Where:

Ref(Z) is the Z transform of the signal recorded at the reference 24;

Bit(Z) is the Z transform of signal generated by the drill bit 18;

T(Z) is the transfer function between the drill bit 18 and the reference24;

N(Z) is the Z transform of noise, both coherent and random, sensed atthe reference 24.

The cross-correlation of the swivel reference 24 signal with a fieldsensor 20 signal, XCOR(Z), can be represented as

    XCOR(Z)=REF(1/Z)Geo(Z)

Where:

Ref(1/Z) is the Z transform of the signal recorded at the reference 24reversed in time;

Geo(Z) is the Z transform of the geophone 20 output.

To achieve the type of broadband source spectrum that is commonlyemployed in reflection seismology it may be necessary to whiten thespectrum of the reference signal. This can be accomplished by deriving awhitening filter based on the reference 24 signal, and convolving thatfilter response with the correlations. The filter response can also beapplied prior to cross-correlation with the same effect. The whiteningfilter derived is an approximation to the inverse of the referencesignal 24 reversed in time. This inverse can be found by any number ofdeconvolution techniques employed in seismic signal processing such aspredictive, maximum likelihood, minimum entropy, or spectralfactorization, as respectively disclosed in:

Peackock, K. L. and Treitel, S., PREDICTIVE DECONVOLUTION, THEORY ANDPRACTICE: Geoph. v. 34, pp. 155-169

Mendel, J. P., SINGLE CHANNEL WHITE NOISE ESTIMATORS FOR DECONVOLUTION:Geoph., v. 43, pp 1-22

Bayless J. W. and Bingam J. O. APPLICATION OF THE KALMAN FILTER, Geoph.,v. 35, pp. 2-23

Ulrych, T. J., APPLICATION OF HOMOMORPHIC DECONVOLUTION TO SEISMOLOGY,Geoph. v. 36, pp. 650-661.

In Z transforms

Step 1: Find 1/Ref(1/Z)

Step 2: Multiply XCOR(Z) by 1/Ref(1/Z)

    1/Ref(1/Z)XCOR(Z)=1/Ref(1/Z)Ref(1/Z)Geo(Z)=Geo(Z)

The effects of reference deconvolution on the cross-correlation areshown in FIGS. 3A and 3B. Reference deconvolution causes the wavelet 70to appear sharper. Spatial and temporal resolution are improved with asharper wavelet. Reference deconvolution also attenuates multipath inthe reference signal 72.

SPATIAL FILTERING OF RIG GENERATED ENERGY

Since drill bit generated vibrations are picked up on the drill rig, itis not surprising that this vibration travels from the drill rig intothe earth. Thus, the drill rig 12 acts as a secondary radiator ofseismic energy originally radiated at the drill bit. As FIG. 4 shows,drill bit generated direct and reflected signals 74 and 76 exhibitcross-correlation delay time variation (commonly referred to as"moveout") with drilling depth. In FIG. 4 a series of cross-correlationfunctions between the rig reference channel 78 and a selected geophonechannel are plotted for various selected drill bit 18 depths. As thedrill bit 18 goes deeper, the travel-time along the direct ray 74increases. Conversely, as the drill bit goes deeper, the travel-timealong the raypath 76 reflected from a given horizon decreases. Thechanges in the respective arrival times creates visible moveouts whichare evident in the Figure. In contrast to these arrivals is theinterference 78 from the drill rig. It is coherent with the usefulprimary energy, but does not exhibit cross-correlation moveout withdrilling depth.

The moveout differences between rig interference 78 and bit generateddirect and reflected arrivals 74 and 76 are exploited to attenuate anyenergy generated by the drill rig 12. Zero moveout energy is equivalentto zero spatial frequency (k=0). A spatial filter designed to attenuatevery low spatial frequency data while passing those events with non-zeromoveout will attenuate rig generated signals 78 while passing, unharmed,bit generated direct and reflected arrivals 74 and 76. The filtering canbe accomplished by any number of spatial filtering techniques, some ofwhich are as follows.

One technique transforms cross-correlated signals from a given geophonegroup and from a series of drilling depths to the domain of"frequency-wavenumber", or f-k, with a two-dimensional Fouriertransform. Different events can then be separated on the basis ofapparent velocity Va, which is a slope in the f-k domains

    Va=f/k.

The filter is designed by specifying slopes as bounds on the passed orrejected regions of the f-k domain. Energy within the passed region isgiven a high weighting coefficient, while energy within the rejectedregion is given a low weighting coefficient. Edge effects of the filtercan be minimized by smoothly interpolating the weighting coefficientsbetween the passed and rejected regions of the f-k domain. The filteringcan be performed as a multiplication in the f-k domain or as aconvolution in the untransformed domain.

Another technique for selectively attenuating or enhancing differenttypes of arrivals begins by time shifting the desired event so that ithas zero moveout. In the special case of rig generated energy, no timeshifting need be done. Filtering is accomplished as follows. First, thearrival aligned to zero moveout is enhanced by averaging a series ofcross-correlations together. This averaging can be done using the meanor the median statistic. The spatial bandwidth of the filter iscontrolled by the number of cross-correlations averaged. More averagingis equivalent to a tighter filter: The output average is the enhancedversion of the aligned zero moveout event. If this is the only goal, thedata can now be de-aligned (if necessary). The zero moveout event can beattenuated by subtracting this average from the original data, followedby dealignment.

Further details on these and other spatial filtering techniques aredisclosed in the following references:

Embree, P., Burg, J.P., Backus, M.M., WIDE-BAND VELOCITY FILTERING THEPIE-SLICE PROCESS, Geoph., v. 28, pp. 948-974

Treitel, S , Shanks, J.L., Francis, C.W., SOME ASPECTS OF FAN FILTERING,Geoph., v. 32, pp. 789-800

Sengbush, R.L., Foster, M.R., OPTIMUM MULTICHANNEL VELOCITY FILTERS,Geoph., v. 33, pp 11-35.

Hildebrand, L., TWO REPRESENTATIONS OF THE FAN FILTER, Geoph., v. 47,pp. 957-959.

That portion of rig generated energy that travels in the near surfacemay also be attenuated by spatial filters that take advantage of themoveout differences between deep events and those that travel in thenear surface. The apparent velocity, V_(a), of an event across astraight line array is well known to be related to the intrinsic earthvelocity and arrival angle by

    V.sub.a =V.sub.intrinsic /cosθ

where θ is measured with respect to the horizontal Deep arrivals fromthe drill bit will generally exhibit higher apparent velocities thannear surface traveling paths. By implementing arrays which attenuate lowapparent velocity energy, near surface traveling interference from thedrill rig can often be attenuated. The filter can be implemented in thefield, in the form of a geophone string containing a number of geophoneseach spaced selected distances apart, or later by mixingcross-correlations from several offsets in the computer.

ACCOUNTING FOR DRILLSTRING DELAY

To properly represent the cross-correlated data as if a referenceproximal to the drill bit were used, the data must be time shifted.Cross-correlated signals using a reference sensor 24 on drill bitgenerated energy located at the top of the drillstring 14 are timeadvanced relative to a reference located at the drill bit. This advanceis equal to the travel-time of waves from the drill bit 18 up thedrillstring 14 to the top of the drillstring on the drill rig 12. Withknowledge of acoustic velocity in the steel pipe drillstring and withknowledge of the drillstring length, the travel-time can be determined.The cross-correlated data is then time shifted to position events as ifthe reference signal on drill bit generated energy were located at thedrill bit 18 itself. Alternatively, the reference signal can betime-shifted prior to cross-correlation.

EXPLOITATION OF DRILL BIT GENERATED DIRECT AND REFLECTED SIGNAL

The aforementioned processing was aimed at arriving at the equivalent ofa controlled downhole seismic source, either impulsive orVibroseis-like. The input to this processing were signals from thedrilling sound as sensed at the surface on the drill rig and in theearth in the vicinity of the drill rig. With a broadband downholeseismic source, direct and reflected signals 74 and 76 can be utilizedto obtain seismic information about the subsurface in the vicinity ofthe borehole using some of the following methods.

WAVEFIELD SEPARATION

In exploiting the drill bit generated signal it is necessary to separatethe data into two subsets: one containing the direct arrival signal 74,the other containing the portion 76 of the signal coming fromreflections beneath the drill bit depth. This process is known aswavefield separation. The processing of VSP data uses wavefieldseparation to separate "upgoing" (reflections) and "downgoing" (directarrivals) signals, as disclosed by Hardage, B., VSP PRINCIPLES, Geophy.Press, pp. 173-194. With a downhole source the terms "upgoing" and"downgoing" must be reversed. In this application we have used the termsdirect and reflected arrivals to avoid confusion.

As FIG. 4 shows, drill bit generated direct and reflected signals 74 and76 can be distinguished in the cross-correlation functions from a singlechannel based on its arrival time moveout with drill bit depth.Wavefield separation processing exploits moveout differences to separatethe data set into the direct arrival signal 74 and reflected arrivalsignal 76. Spatial filters are used to pass or reject differentmoveouts. For example, a filter which passed positive moveouts(cross-correlation arrival times which increased with drill bit depth)would enhance the direct arrival 74 while attenuating signals 76 fromreflecting layers which have negative moveout. Conversely, a secondfilter passing negative moveouts and rejecting positive moveouts wouldenhance reflected signals 76 while attenuating the direct arrival signal74. Alternatively, the direct arrival 74 can be enhanced by passing arange of positive moveouts. The enhanced direct arrival signalrepresents one data subset. This subset may be subtracted from theoriginal data to yield the remaining signal. The spatial filter can be amix of adjacent traces, a pie-slice filter applied in the f-k domain, orany other spatial filtering technique. FIG. 3A shows the original data,FIG. 5A shows the wavefield separated direct arrival 74, and FIG. 5Bshows the residual data 80 containing the reflected arrivals. Furtherprocessing and stacking is often necessary to enhance the reflectedarrivals.

DIRECT ARRIVAL UTILIZATION

The direct arrival 74 from the drill bit 18 holds important informationunavailable from surface seismic data. Propagation velocity is availablefrom the direct arrival signal 74.

THE DIRECT ARRIVAL FOR PROPAGATION VELOCITY DETERMINATION

The travel-time for energy from a given drill bit depth to reach theearth's surface via a direct path divided into the distance from the bitto the surface location is equal to the average velocity for thatdistance. The travel-time difference from two different drill bit depthsdivided into the differential distance is equal to the interval velocitybetween these two drill bit depths (the travel-path from the drill bit18 to the earth's surface may be complicated somewhat by refraction ofwaves at interfaces, but if the arrivals are near vertical, this effectwill be small). The average velocity allows two-way travel-times of nearhorizontal events on surface seismic data to be converted to subsurfacedepth. Knowledge of reflector depths provides information which can aidboth the explorationist and the driller in many ways. For example, witha knowledge of the depth to a target horizon, a better forecast ofdrilling time and expenditures can be made. Knowledge of depths ofoverpressured zones can be of crucial importance during drilling.

Interval velocities can be used to identify stratigraphic units. Theyare also used in imaging techniques that require an accurate knowledgeof wave propagation in the earth.

IMAGING USING DRILL BIT GENERATED DIRECT AND REFLECTED SIGNALS

Imaging using the drill bit source differs from conventional surfaceseismic imaging because one leg of the travel-path of the reflectedarrival is significantly shorter than the other. This phenomena is alsothe reason for the improved lateral resolution using a downhole sourcecloser to reflecting horizons of interest. In surface seismic data, bothlegs of the travel-path are approximately the same. Conventional CDPstacking algorithms exploit this geometry by grouping all data that isequidistant between depth or common reflection point, as described inMayne et al., U.S. Pat. No. 2,732,906. This approximation is accuratefor a flat layered earth.

With a drill bit signal, different source 18 and receiver 20 pairs havea common reflection point 82 that is no longer halfway between source 18and receiver, as shown in FIG. 6. This means that the transformation isarrival time dependent. Different reflections from the same sourcereceiver pairs will have different Common Reflection Points.Cross-correlations from selected drill bit depths and surface geophonelocations are associated with arrival times from common reflection pointfocus depths. The arrival times are calculated by raytracing from sourceto reflector to receiver through a given velocity model,Cross-correlations from each focus depth are then time shifted (toaccount for delay differences) and summed. The Common Reflection Pointtransformation is described in terms of a VSP geometry by Wyatt, K.D.and Wyatt, S.B., THE DETERMINATION OF SUBSURFACE STRUCTURAL INFORMATIONUSING THE VERTICAL SEISMIC PROFILE, Technical Paper No. 55.2, 51stAnnual Meeting of SEG, pp. 1915-1949. FIG. 7 shows an example of atransformation of the drill bit generated reflection data to CommonReflection Point 84. The transformation assumed a horizontallystratified earth. Reflecting layers are clearly imaged using the drillbit signal.

By including refraction of waves at interfaces, dipping reflectors, andraytracing, the transformation becomes more than a "CDP stack", itbecomes a ray-theoretically correct imaging technique.

It is very important in common reflection point imaging that thevelocity function of the region be accurately known. This is sometimestermed the "chicken and the egg" constraint on imaging of surfaceseismic reflection data. To image the data correctly, the velocityfunction in the earth must be accurately known, but to determine thevelocity function it is usually necessary to have an imaged seismic dataset. Fortunately, our data circumvents this problem with the intervalvelocities obtained from the direct arrival.

With conventional surface seismic, the seismic interval velocities arederived indirectly through either external information .g., sonic logs,or checkshot information nearby), or through velocity analysis of theacquired data. With the drill bit signal, interval velocities can bedirectly determined through the correlation time delay of the directarrival signal. This yields accurate velocities above the drill bitdepth. Below this depth extrapolation of other information or velocityanalysis must be used as in surface seismic data.

It should now be readily apparent to those skilled in the art that anovel apparatus and method for providing information while drilling onsubterranean geologic formations capable of achieving the stated objectsof the invention has been provided. The apparatus and method makeimproved use of a reference signal to provide the desired information.Improved signal processing of the seismic wave signals is made possibleby the reference signal. All of the seismic wave signals used to obtainthe desired information originate at the drill bit. The apparatus andmethod is non-invasive and generates the signals used to obtain thedesired information while drilling. The apparatus and method providesreal time subsurface images.

It should further be apparent to those skilled in the art that variouschanges in form and details of the invention as shown and described maybe made. It is intended that such changes be included within the spiritand scope of the claims appended hereto.

What is claimed is:
 1. An apparatus for providing information on asubterranean geologic formation, which comprises a drill bit source ofenergy in a borehole, at least one seismic wave sensor spaced from theborehole in the earth for receiving seismic waves imparted into theearth by said drill bit source of energy, a reference sensor foracoustic waves imparted into the borehole by said drill bit source ofenergy, a means connected to receive a reference signal from saidreference sensor and a signal from said at least one seismic wave sensorsimultaneously, recorded during a period of time when said drill bitsource of energy is activated at a selected source depth and to generatea cross-correlation function between the reference and seismic wavesensor signals, a means connected to receive the reference signal toderive a whitening filter, and a means connected to receive the filterfor removal of the combined effects of reference signal source spectralshaping and multipath from the cross-correlation function, the whiteningfilter being derived as an inverse to the reference signal and beingapplied to the cross-correlation function by time-reversing and thenconvolving the filter with the cross correlation function oralternatively being applied by time-reversing the filter and convolvingthe filter with the time-reversed reference signal prior tocross-correlation, said apparatus being configured to carry out itssignal processing operations repetitively for a series of source depths,generating a series of filtered cross-correlation functions, saidreference sensor being located at or proximate to a top of the boreholedistant to said drill bit source of energy.
 2. The apparatus forproviding information on the subterranean geologic formation of claim 1in which said reference sensor comprises an accelerometer located on aswivel of said drilling rig.
 3. The apparatus for providing informationon the subterranean geologic formation of claim 1 in which said drillingrig is located on an offshore platform.
 4. The apparatus for providinginformation on the subterranean geologic formation of claim 1 in whichsaid at least one seismic wave sensor comprises a plurality of sensorslocated proximate to a surface of the earth at predetermined locationsrelative to the borehole.
 5. The apparatus for providing information onthe subterranean geologic formation of claim 1 in which incoherentenergy is attenuated in the cross-correlation function.
 6. The apparatusfor providing information on the subterranean geologic formation ofclaim 1 additionally comprising means for converting the travel times ofarrivals represented in the cross-correlation function to travel timesthat would be observed if a reference sensor were located proximate tosaid source of energy by time shifting the cross-correlation function,the time shift being made equal to the delay of acoustic waves travelingfrom the source location to the reference location at the top of theborehole, the delay being calculated as the length of the acoustic pathdivided by the acoustic velocity along said path.
 7. The apparatus forproviding information on the subterranean geologic formation of claim 6additionally comprising means connected to the cross-correlated andtime-shifted signals for determining interval velocities based ondifferential travel time of the direct arrival from two different sourcedepths divided into a differential path length from said source to saidat least one seismic wave sensor.
 8. The apparatus for providinginformation on the subterranean geologic formation of claim 6additionally comprising means connected to receive the cross-correlatedand time shifted signals for determining average velocities between thesource depth and the sensor location at the surface based on the traveltime of the direct arrival signal divided into the distance from thesource location to the geophone location.
 9. The apparatus for providinginformation on the subterranean geologic formation of claim 6additionally comprising a means connected to receive the series ofcross-correlation functions for separating the series ofcross-correlation functions into a first group representing the seismicwaves received by said at least one seismic wave sensor via directseismic wave paths, and a second group representing the seismic wavesreceived by the at least one seismic wave sensor via seismic wave pathsreflected by the subterranean geologic formation.
 10. The apparatus forproviding information on the subterranean geologic formation of claim 9in which said separating means is configured to attenuate direct arrivalenergy in the series of cross-correlation functions from differentsource depths by averaging the cross-correlations and subtracting theaverage from the series of cross-correlation functions from differentsource depths.
 11. The apparatus for providing information on thesubterranean geologic formation of claim 9 in which said separatingmeans is configured to transform the cross-correlation functions from aseries of drilling depths to a domain of frequency-wavenumber with atwo-dimensional Fourier transform and then attenuate direct arrivalenergy through the use of a pie-slice spatial frequency attenuationfilter.
 12. The apparatus for providing information on the subterraneangeologic formation of claim 9 additionally comprising a means connectedto the first group of the separated cross-correlation functions toderive a deconvolution operator, a means connected to the second groupof separated cross-correlation functions and to said deconvolutionoperator deriving means to deconvolve the second group of the seismicwave sensor signals.
 13. The apparatus for providing information on thesubterranean geologic formation of claim 9 additionally comprising meansconnected to the cross-correlated and time-shifted signals for providinga common reflection point image of the subterranean geologic formation.14. The apparatus for providing information on the subterranean geologicformation of claim 13 in which said means for providing the commonreflection point image is connected to receive the second group ofcross-correlation functions representing the seismic waves reflected bythe subterranean geologic formation for providing the common reflectionpoint image.
 15. The apparatus for providing information on thesubterranean geologic formation of claim 14 in which said means forproviding the common reflection point image is configured to provide theimage from a transformation via a ray tracing procedure combiningreflected energy from a given reflection point in the earthcorresponding to different source-receiver pairs.
 16. A method forproviding information on a subterranean geologic formation, whichcomprises providing a drill bit source of energy in a borehole,receiving, with at least one seismic wave sensor spaced from theborehole in the earth, signals representative of seismic waves impartedinto the earth by the drill bit source of energy, simultaneouslyreceiving a reference signal from acoustic waves imparted into theborehole by the drill bit source of energy, the signals being receivedduring a period of time when the drill bit source of energy is activatedat a selected source depth, generating a cross-correlation functionbetween the reference and the seismic wave sensor signals during aperiod of time when the source of energy is activated at the selectedsource depth, deriving a whitening filter from the reference signal thewhitening filter being configured as an inverse to the reference signal,applying the filter to the cross-correlation function by time-reversingand then convolving the filter with the cross-correlation function oralternatively applying the filter by time-reversing the filter andconvolving the filter with the time-reversed reference signal prior tocross-correlation for removal of the combined effects of referencesignal source spectral shaping and multipath from the cross-correlationfunction, and repeating the above operations for the series of sourcedepths, generating a series of filtered cross-correlation functions, thereference signals being received at or proximate to a top of theborehole distant to the drill bit source of energy.
 17. The method forproviding information on the subterranean geologic formation of claim 16in which the signals containing seismic waves are received at aplurality of predetermined locations relative to the borehole proximateto a surface of the earth.
 18. The method for providing information onthe subterranean geologic formation of claim 16 in which the signalscontaining seismic waves are received simultaneously at a plurality oflocations in the earth spaced from the borehole and in which incoherentinterference is attenuated by combining the signals containing theseismic waves and by the cross-correlation process.
 19. The method forproviding information on the subterranean geologic formation of claim 16in which the borehole contains the drill bit source of energy attachedto a drill string in the borehole and the acoustic waves received by thereference sensor propagate within the drill string.
 20. The method forproviding information on the subterranean geologic formation of claim 16in which the reference signals are received at the top of the borehole,the method additionally comprising the step of converting the traveltimes of arrivals represented in the cross-correlation function totravel times that would be observed if a reference sensor were locatedproximate to the drill bit source of energy by time shifting thecross-correlation function, the time shift being made equal to the delayof acoustic waves traveling from the drill bit source of energy to thereference location at or proximate to the top of the borehole, the delaybeing calculated as the length of the acoustic path divided by theacoustic velocity along the path.
 21. The method for providinginformation on the subterranean geologic formation of claim 20 in whichthe time delay is applied to the reference signal prior tocross-correlation.
 22. The method for providing information on thesubterranean geologic formation of claim 20 additionally comprising thestep of determining interval velocities from the cross-correlated andtime-shifted signals based on the differential travel time of the directarrival from two different source depths divided into a differentialpath length from the drill bit source of energy to the seismic wavesignal receiving location.
 23. The method for providing information onthe subterranean geologic formation of claim 20 additionally comprisingthe step of determining average velocities between the source depth andthe sensor location at the surface from the cross-correlated and timeshifted signals based on the travel time of the direct arrival signaldivided into the distance from the source location to the seismic wavesignal receiving location.
 24. The method for providing information onthe subterranean geologic formation of claim 20 additionally comprisingthe step of separating the cross-correlation functions into a firstgroup representing the seismic waves received by direct seismic wavepaths, and a second group representing the seismic waves received viaseismic wave paths reflected by the subterranean geologic formation. 25.The method for providing information on the subterranean geologicformation of claim 24 additionally comprising the step of attenuatingdirect arrival energy in the series of cross-correlation functions fromdifferent source depths by aligning in time the direct arrival energy ofthe series, and by averaging the cross-correlations and subtracting theaverage from the series of cross-correlations from different sourcedepths.
 26. The method for providing information on the subterraneangeologic formation of claim 24 additionally comprising the steps oftransforming the cross-correlation functions from a series of drillingdepths to a domain of frequency-wavenumber with a two-dimensionalFourier transform and then attenuating direct arrival energy through useof a pie-slice spatial frequency attenuation filter.
 27. The method forproviding information on the subterranean geologic formation of claim 24additionally comprising the steps of deriving a deconvolution operatorto deconvolve earth reverberations from the first group of the separatedcross-correlation functions and using the deconvolution operator todeconvolve the second group of the separated cross-correlationfunctions.
 28. The method for providing information on the subterraneangeologic formation of claim 24 additionally comprising the step ofproviding a common reflection point image of the subterranean geologicformation from the cross-correlation and time-shifted signals.
 29. Themethod for providing information on the subterranean geologic formationof claim 28 in which the second group of cross-correlation functionsrepresenting the seismic waves reflected by the subterranean geologicformation is used to provide the common reflection point image.
 30. Themethod for providing information on the subterranean geologic formationof claim 29 in which the common reflection point image is provided froma transformation via a ray tracing procedure combining reflected energyfrom a given reflection point in the earth corresponding to differentsource-receiver pairs.